Depending on the type of material the processing may involve mixing, kneading, and/or melting. E.g. thermoplastic materials, in particular polymers or polymer blends, can be molten in a heated extruder and shaped in the molten state, e.g. as test samples. In the same process additives may be mixed in. Other, non-thermoplastic materials, e.g. food or other edible materials, may be mixed with other components in the extruder and as a mixture be given a desired shape, e.g. a film and granules.
In particular extruder according to the invention is an extruder having a limited size and effective volume, making it suitable for melting and mixing polymers that are available only in small amounts. Their throughput may range from 0.05 g/min to some 10 g/min or, in case the extruder is a batch type one, their effective volume may range from 5 to 50 ml. Extruders of this type may be particularly applied in a laboratory environment for processing experimental material grades ands mixtures, which use to be available only in small quantities, into test samples. Available quantities may range from 50 g down to 15 g and even only 5 grams. The shaping into test samples is carried out in a manner known from industrial extruders by feeding the mixed and/or molten material to a downstream exit of the extruder bore through a die or into a mould, each of a proper shape.
Such an extruder is known from U.S. Pat. No. 4,695,240. This publication teaches that small extruders cannot easily be constructed by downscaling large-scale industrial extruders. To achieve sufficient residence time the extruder barrels would have to be relatively long, whereas the extruder screw then becomes relatively thin. Screw failure due to the high torsion forces caused by the changing viscosity of the polymer to be processed is to be expected. The problems associated with downscaling are resolved there by introducing a heat separation zone between the Inlet and the outlet portion of the extruder bore. It is alleged that their construction eliminates the problem of premature failure to the extruder screws associated with scaled down conventional extruder apparatus. In the document, however, it is not disclosed how the proposed measure does eliminate the problem.
Aim of the invention is to provide an extruder, having a solid construction and suitable to process a variety of small available amounts of polymer.
This aim is achieved according to the invention in that the at least one extruder screw has a length:diameter ratio of less than 20:1, in that the extruder further comprises at least one recycling channel running through the barrel from the outlet portion of the bore and at least two recycling exits that connect a recycling channel to the bore upstream of the outlet portion, and in that it further comprises a flow control system for directing the material through the at least one recycling channel and the recycling exits and/or the extruder exit.
Thus in the extruder according to the invention the at least one screw is favorably solid whereas by directing the material stream in the extruder through the recycling channel(s) to specific recycling exits the effective volume of material that can be cycled through the extruder can be varied, adapting the extruder to the amount of material available.